SPOEES AND THALLIDIA. 13 



surface of the frond, for this is of very uncommon occurrence. They are usually 

 situated on the under surface of the frond, the reason being that they are thus best 

 sheltered from both rain and sun. Most instances exhibit in addition a further 

 safeguard against excessive moisture or desiccation in the form of a special awniug 

 covering the sporangia. This awning is either an outgrowth from the cells forming 

 the apex of the sporangiferous cushion or peg, and takes the form of a delicate 

 membrane stretched over the whole sorus and known as an indusiv/m, as in 

 our male Shield Fern (AspidiuTn Filix-^mas), or else small, scale-like leaflets 

 spread themselves over the sporangia, as in the Gleichenias (fig. 189*), to which 

 reference has already been made, and in the no less remarkable Lygodiums and 

 Davallias. Sometimes five or six squamous leaflets stand in a circle round the 

 sporangia and envelope them so that the whole looks deceptively like a closed 

 flower, as ia the genera SchizocoBna, Mymenocystis, and BiacaVpe; or, these leaflets 

 form a sort of box, which opens with a Hd, as in Cihotium,. In other cases, again, 

 membranous bands or borders grow up from the surface of the frond, and the 

 sporangia, which are arranged in a long line, are covered over by them, as occurs 

 iu Lvndsaya and Blechnv/m; or, the margin of the frond is split and the sporangia 

 are hidden in the narrow cleft thus made, as in Yittaria and Schizoloma. Often 

 the margin of the frond is folded over, thus covering the sporangia, which are here 

 developed on raised cushions; Allosorus, Ceratopteris, Ceratodactylis, Parkeria, and 

 many other genera exhibit this formation. The extreme variety prevaihng in this 

 class of adaptation is connected with differences in the climatic conditions of the 

 habitats where the plants in question live. Any attempt to deal with individual 

 contrivances here would lead us too far. 



The Rhizocarpeae are a group nearly allied to Ferns, and they naturally follow 

 the same lines as Ferns in the formation of their spores and sporangia. Salvinia 

 reminds one to some extent of the Hymenophyllaceae, at any. rate as regards the 

 outgrowth of an annular wall below the sporangia (the latter being in this case also 

 borne on a fusiform axis), and also as regards the development of this wall, which 

 becomes closed at the top into a complete box enshrouding the sporangia. Marsilea, 

 on the other hand, exhibits stalked, bean-shaped capsules with cavities in which 

 the sporangia are formed on raised cushions. 



The Club-mosses (Lycopodiaceae) also bear a striking resemblance in their mode 

 of spore-formation to Ferns, especially to the various species of Lygodiv/m and 

 Lygodictyan, genera of which mention has already been made. The first rudiments 

 of the sporangia are swellings at the base of the little squamiform leaves, or on the 

 axis just at their insertion. The internal tissue of this protuberance is marked off 

 in the form of a roundish ball. The cells constituting the baU separate and then 

 become segmented each into four chambers, the walls of which are subsequently 

 dissolved. The protoplasts within the chambers inclose themselves in membranes 

 and become free spores. The epidermis originally clothing the swelling persists, 

 and now forms the wall of a bean-shaped sporangium containing loose spores. 

 The sporangium subsequently opens by means of a lid like a box. 



